Cannabinoids in the brain: their mechanism of action in relation to neuropsychiatric disorders

Understanding how the endogenous cannabinoid system operates laid foundations to rationalizing the action of the major plant-derived cannabinoids, ∆⁹-tetrahydrocannabinol and cannabidiol. Here, the progressive emergence of plant-derived cannabinoids in medical practice is illustrated by their potential relevance to the treatment of schizophrenia and epilepsy.

Introduction: a tale of recreational use and therapy

Evolutionary biology posits the existence of Cannabis sativa for the last 600 million years. Co-evolution with humans was because of the plant’s richness in durable fiber rather than its perceived psychotropic significance. The first mention of Cannabis sativa in relation to a discrete medical condition, pain, was in ancient China ~4000 B.C., and included detailed knowledge on the plant’s psychotropism, which suggests a significant content in ∆⁹-tetrahydrocannabinol (THC). During the ensuing millennia, medical benefits of Cannabis spp. were repeatedly mentioned by Greeks, Romans, Mongols and others. Written records show the widespread use of cannabis extracts for medicinal purposes by the 19^th century. In 1937, however, its assumed abuse potential led to the banning of marijuana from further medicinal use in the United States. The beginning of a substantial and progressive shift of research, medical and legislative positions on plant-derived cannabinoids can probably be pinned to 1967, when THC was isolated and chemically characterized^1,2, and recognized as the main psychoactive constituent of Cannabis sativa.

Subsequent chemical analyses showed that THC is just one of the many bioactive compounds in the host plant: by now >100 cannabinoid-family compounds (collectively referred to as “phytocannabinoids”), as well as many other substances (therpens, flavonoids) that might also produce marked bodily responses in humans are distinguished. By broad definition, phytocannabinoids are separated into groups of “drug-type” and “fiber-type” substances. The former group includes THC and its derivatives (tetrahydrocannabivarin, THC-acid) with psychoactive effects. The latter group, in turn, is a conglomerate of non-psychoactive cannabinoids like cannabidiol (CBD), cannabigerol and cannabidivarin. The rapid expansion of knowledge on phytocannabinoids together with the increasing social acceptance of cannabis as a moderately addictive drug in adult users generated significant interest in both phytocannabinoid-based and synthetic cannabinoid therapies (for, e.g., pain, osteoporosis, neuropathies, neurodegeneration, epilepsy, inflammation and cancer).

Enrichment in cannabinoids by selective cultivation

Originally, herbal cannabis contained minute amounts of psychoactive constituents, which accumulated in floral oils likely to repel insects. To meet the demand of increasing cannabis consumption for its psychoactive potency worldwide, significant advances in selective cultivation and hybrid production ensued. This, together with improved extraction and preparative procedures, helped to maximize the THC (but also CBD) content. Worryingly, herbal mixtures that contain powerful synthetic additives (such as “Spice”) are increasingly abused alike new generations of synthetic cannabinoids that are ~10-200x more potent than THC and typically do not contain CBD.^3,4

Cannabinoid receptors as molecular targets for THC and related ligands

The effects of THC (and many others cannabinoids but CBD) are chiefly mediated by two members of the G protein-coupled receptor (GPCR) family: cannabinoid receptor 1 (CB₁R) and its orthologue, CB₂R. Both receptors recruit G_i/G₀ proteins, whose Gαi subunit inhibits adenylate cyclase. CB₁Rs also modulate ion channels, inducing the inhibition of voltage-sensitive Ca²⁺ channels and activation of G protein-activated inwardly-rectifying K+ channels.^5,6 This mechanism is of primary importance in excitable tissues and leads to the inhibition of exocytosis upon cell depolarization (that is, negative regulation of neurotransmitter release and hormone secretion in the nervous system and at the periphery, respectively). CB₁R/CB₂R, via Gβγ, also modulate pleiotropic intracellular signaling cascades, including mitogen-activated protein kinases (MAPKs)/extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (Jun), p38 and phosphatidylinositol 3-kinase (PI3K)/Akt. These systems are fundamental to the regulation of cell proliferation, cell cycle control and cell death. Moreover, CB₁Rs regulate cellular metabolic processes through the intracellular release of ceramide by interaction with neutral sphingomyelinase via FAN.⁷ Besides CB₁R and CB₂R, endocannabinoid (and phytocannabinoid) action is transduced by a variety of alternative receptors, including TRPV1, GPR55 and PPARs. In contrast to THC, CBD action is considered largely non-CB₁R-mediated. Even if alternative cannabinoid receptors (TRPV1, GPR55, PPARs) might play a role, the enhancement of antioxidant defense and direct action on mitochondrial bioenergetics are favored.

CB₁Rs are the most abundant GPCRs in the mammalian brain, primarily expressed in neurons and to a lesser extent in astroglia.⁸ CB₁Rs concentrate in areas controlling motor, cognitive, emotional and sensory functions (e.g., the hippocampus, basal ganglia, cerebellum, cerebral cortex and olfactory bulb).^9,10 Foci of CB₁Rs are also found in areas controlling pain (brainstem), body temperature, sleep-wake cycles (hypothalamus), and hormone secretion (pituitary).¹¹ Beyond the nervous system, CB₁R expression was reported in the eye, muscle, pancreas, heart, lung,^12-15 placenta and myometrium,^16-18 highlighting its role in body-wide homeostatic control.

CB₂Rs are classically associated with cells of the immune lineage. Within the brain, CB₂Rs were unequivocally detected in perivascular microglia and vascular endothelial cells. CB₂Rs in neurons (particularly in the brainstem) remain a contentious issue.¹⁹ Increased CB₂R expression, possibly owing to infiltration of immune cells and activation of microglia, is recognized in certain neurodegenerative disorders.²⁰

Mechanism of action: THC impairs endocannabinoid signaling

The discovery of cannabinoid receptors led to the rapid identification of a family of small unsaturated fatty acids (eicosanoids) as their natural ligands.^21-23 2-arachidonoylglycerol (2-AG) and anandamide (AEA) are the best-characterized endocannabinoids. Endocannabinoids are produced primary “on demand” from membrane lipid precursors by multiple biosynthetic pathways²⁴ and reach their receptors by (facilitated) diffusion. AEA is formed by the cleavage of its phospholipid precursor, N-arachidonoyl-phosphatidylethanolamine (NArPE).^25,26 By contrast, sn-1-diacylglycerol lipase (DAGL) α/β are responsible for the synthesis of 2-AG.²⁷ Endocannabinoid signaling is terminated by enzymatic hydrolysis: fatty acid amide hydrolases (FAAH1/2) inactivate AEA,^28,29 while monoacylglycerol lipase (MAGL) and ABHD6 catabolize 2-AG.^30-32 Moreover, AEA can be oxygenated by cyclooxygenase-2 (COX-2), 5-,12- and 15-lipoxygenase (5-/12-/15-LOX), and by several cytochrome P450 monooxygenases.²⁹

In the adult brain and spinal cord, the signaling paradigm for endocannabinoids is their activity-dependent (Ca²⁺-dependent) short-lived production in postsynaptic neurons. This is followed by endocannabinoid diffusion (in a 20-100 µm radius) towards presynaptic terminals endowed with CB₁Rs, followed by signal transduction. Upon endocannabinoid binding, CB₁Rs internalize and endocannabinoids become rapidly inactivated by hydrolysis.³³ Endocannabinoid signaling in the developing nervous system is also CB₁R dependent, and primarily associated with stem cell renewal, cell migration and axonal pathfinding.³⁴ In both cases, THC can produce unwanted CB₁R occupancy for periods longer than produced by physiological endocannabinoid signals, thus changing the timing of both synaptic neurotransmission and neurocircuit maturation in adult and developing brain, respectively.

Long-lived THC effects during pre- and postnatal brain development

Endocannabinoids control at least 6 stages of cellular development in the brain: i) stem cell division, ii) primary fate decisions of neuronal progenitors, iii) survival of progeny, iv) neuronal migration, v) neuronal morphogenesis with particular impact on neurite outgrowth and vi) synaptogenesis.^35-37 To orchestrate stages iv-vi, CB₁Rs are localized on the surface of distal axons segments^35,38 including growth cones to modulate directional steering decisions.^39,40 The presence of CB₁Rs on elongating neurites explains why THC is particularly detrimental for ordered brain connectivity to evolve. Given that THC efficiently crosses the placenta,^41,42 is detectable for hours to several days and lipophilic, maternal THC intake can be powerful to disrupt the temporal precision of physiological endocannabinoid action. Even more so, THC is not inactivated by any of the known endocannabinoid hydrolases. Thus, THC can engage CB₁Rs prematurely and even intracellularly, thus compromising neuronal morphogenesis, hindering synaptogenesis and, consequently, limiting synaptic neurotransmission. Indeed, experimental data demonstrate that THC administration in vivo perturbs neuronal connectivity in mouse fetuses and infant offspring^38,43 through CB₁R engagement.^34,44-46

The refinement of neuronal circuits continues during adolescent life with synaptic selection and pruning taking place to finalize synaptic connectivity in neuronal networks. Thus, cannabis use by teenagers can, at least partly, (re-)activate the mechanisms discussed for fetal development. Alternatively, THC can perturb cellular bioenergetics in neurons particularly by disrupting complex I of the mitochondrial respiratory chain,⁸ an adverse effect seen as long as 4-5 months after THC administration, at least in juvenile laboratory rodents.⁴⁷ Thus, a second punchline for THC to impair brain development is through depleting cellular energy reserves to limit neuronal survival and function.

Endocannabinoid signaling in schizophrenia

High-potency cannabinoids increase the incidence of physiological (e.g., nausea) and psychiatric adverse outcomes (e.g., anxiety and psychosis). Notably, a correlation exists between first-time cannabis use during the individual’s life and the severity and relapse potential of cannabis action.⁴⁸ Thus, it is becoming broadly accepted that children and adolescents are significantly more sensitive to cannabis, either directly or in transgenerational settings, and carry the risk of life-long impact on cognition, academic performance, impulsivity and social interactions.

One of the psychiatric disorders that has the clearest association with cannabis use is schizophrenia, a debilitating chronic disorder that exacts enormous personal, social and economic costs. Epidemiological studies have detected a 1.8-to-3.1-fold increase in the incidence of schizophrenia upon early cannabis use, identifying it as a major trigger for the disease to manifest.⁴⁹ Conversely, cannabis is the drug of choice by schizophrenics despite its potential to exacerbate pre-existing psychotic symptoms or to trigger their re-emergence. Traditionally, schizophrenia is linked to alterations in the reward circuit including its midbrain (dopamine) and cortical components. In post-mortem studies, CB₁R expression is increased in the prefrontal⁵⁰ and cingulate cortices⁵¹ of patients with schizophrenia. Genetic evidence at the level of single nucleotide polymorphisms also associates CB₁Rs (and even CB₂Rs) with an increased likelihood of developing schizophrenia.^52-54 CB₁R polymorphisms were even used as predictors of changes in brain structure (e.g. white matter volume), thus qualifying as a gene-to-environment risk factor for the increased incidence of schizophrenia. It is noteworthy that these association studies are often population-specific with equally strong negative data reported elsewhere (see, e.g., Ref.⁵⁵). Another correlate supporting a role for altered endocannabinoid signaling in schizophrenia is the increased bioavailability of AEA in the cerebrospinal fluid and blood of antipsychotic-naïve first-episode patients,^56,57 which normalizes following antipsychotic treatment. The above data from human studies are also supported by animal models in which modifications to endocannabinoid signaling within the dopamine reward circuit (e.g., producing a striatal “hyperdopaminergic” phenotype at D1 and D2 dopamine receptor-expressing neurons that also harbor CB₁Rs)⁵⁸ behaviorally manifest as schizophrenia-like traits.

There is consensus that THC has no favourable effect for psychiatric disorders. Instead, CBD might be of medical use,^59-66 given its non-psychotropic nature,⁶⁷ with an emphasis on its non-CB₁R-dependent neuroprotective properties instead⁶⁸ (considering that CB₁R modulation itself seems of limited benefit in schizophrenia)⁶⁹. A major label to use CBD is because of its anxiolytic property (that is, treatment of a co-morbidity in schizophrenia) as shown in both animal models⁷⁰ and humans.^71-73 As such, CBD treatment improves serotonin neurotransmission,⁷⁴ particularly at the level of serotonin 1A receptors,^75,76 thus attenuating both stress responsiveness and impulsivity.^74,77-79 Another indication for CBD use in schizophrenics and other heavy cannabis users is its ability to alleviate cannabis withdrawal symptoms,^80-84 thus being medically preferred to treat addiction.

THC and CBD in neurological conditions

THC (and CBD) appears to have clinical benefit in neurological conditions associated with neuronal hyperexcitability and runaway network activity, such as epilepsy and pain. Mechanistically, THC can significantly suppress excitatory neurotransmission by engaging CB₁Rs to prevent, e.g., seizure activity or pain-induced hypersensitivity (hyperalgesia). The market-leading example is Sativex® (a near-equivalent THC:CBD mixture) for the treatment of pain and spasms associated with multiple sclerosis. More recently, Epidiolex® (CBD only) received FDA approval for the treatment of severe epilepsy in children (e.g.,with Lennox-Gastaut and Dravet syndromes).⁸⁵ The suggested mechanism of action for CBD rests largely on dampening neuronal excitability via engagement of TRPV1, GPR55 and likely the ENT-1 nucleoside transporter, resulting in significant seizure reduction.⁸⁶

Summary

The medical use of Cannabis sativa stretches over millennia. However, it is fundamental research of the past ~50 years that has uncovered the molecular and cellular bases of phytocannabinoid action in the human body, and allowed us to rationalize adverse effects vs. medical benefits. Even though specific recommendations already exist for the medical use of THC and CBD, on-going (pre-)clinical research will continue to explore specific indications, as well as labels of use for these powerful natural compounds, supporting their relevance to modern medicine.

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